RATE OF GLOBAL EROSION FROM TECTONIC DIFFUSION

Understanding of relations between global rates of tectonic uplift and continental denudation is an important challenge to a wide range of Earth scientists; global erosion rates have now been empirically constrained from estimates of elevation changes due to tectonic uplift and landscape denudation, global sedimentary rock volumes, and modern river loads. Here, we describe a theoretical link between vertical orogenic tectonism and associated erosion and that is analogous to diffusion of isotopes and elements in minerals. The computational approach uses data on ages and depths of mineralization for epithermal Au-Ag, porphyry Cu, granitic Sn, and orogenic Au hydrothermal ore deposits, and thermochronometric closure depths and ages of fission track data from apatites and zircons, of K-Ar data from amphiboles, biotites, feldspars, and muscovites, of U-Pb data from zircons. The approach is predicated on the presumption that globally-averaged tectonic processes serve to vertically displace various types of lithosomes relative to the Earth’s erosional (absorbing) surface, and the depth/time paths of these rock bodies are largely independent (random). As such, the global population of crustal rock lithosomes can be treated as a random walk system in crustal depth-geologic time space. Tectonic diffusion rates calculated from thermochronometric and hydrothermal ore deposit data show that crustal lithosomes diffuse tectonically at rates similar to those of ions through water, indicate that rate of vertical tectonic dispersion increases with depth in the crust, and require a mean vertical displacement rate of ~200 m/Ma. Given a global sediment flux ~15 Gt/yr, this vertical rate requires uplift over ~20 x 106 km2 (~15%) of continental surfaces; an area the same as that with a mean local relief in excess of 500m represented by active or recently active (eroding) orogens of the world. As a first approximation, the erosional flux of sediment to world oceans is the same as that anticipated from ages and areas of exposed crustal lithosomes.